Methods of switching a load between alternate ac sources have been proposed and tried.
One such attempt to switch a load between alternate sources involved the use of a single electro-mechanical relay to do the source switching. For inductive loads during switching, such single relay systems will, upon movement of the relay armature, produce an arc between the contact on the armature and the particular power source attempted to be disconnected. If sufficient time has not expired when the armature contact engages the other power source, the arc will not become extinguished. Thusly, if the two power sources are not both of the same frequency and the same phase, they will be connected in a serial manner for a sufficient period of time to allow large interpower currents to be produced which undesirably results in the circuit breakers for the respective sources tripping out. Alternatively, producing a slow enough armature travel time to ensure the arc will be extinguished when the armature contact engages the other power source requires a relay of rather large physical size when operating at normal ac power frequencies.
Another such load switching technique is found in U.S. Pat. No. 3,638,040 (Mahmoud) and involves the use of a pair of controlled power transistors to perform the switching. During the switching operation, both transistors are controlled in a manner so that as one transistor is turning off, the other is turning on. Consequently, sufficient series resistance is presented between the two sources during switching to limit to an acceptable value the interpower currents produced, thereby ensuring neither source's circuit breaker will trip. The method of Mahmoud, however, suffers from serious disadvantages. Firstly, the Mahmoud triggering circuitry for the two transistors is complex because Mahmoud must ensure that both transistors are never turned on at the same time. Secondly, the amount of power that can be transferred with the Mahmoud device is limited by the ratings of the switching transistors.
Notwithstanding the switching techniques discussed above, it is apparent the need exists for a less expensive, smaller and higher power device to perform these switching tasks.